The invention is related to a transmission system with a transmitter comprising a time dependent analysis filter bank for deriving from at least one input signal at least two filtered signals, encoding means for deriving encoded signals from the filtered signals, and transmitting means for transmitting the encoded signals to a receiver via a transmission medium, said receiver comprising decoding means for deriving said filtered signals from the encoded signals and a time dependent synthesis filter bank for reconstructing said at least one input signal from the filtered signals.
The invention is also related to a transmitter, a receiver a coder, a decoder and a method for coding and decoding.
A transmission system according to the preamble is known from the conference paper "Exact Reconstruction Analysis/Synthesis Filter Banks with Time Varying Filters" by J. L. Arrowood Jr. and M. J. T. Smith as published in the conference proceedings of the 1993 IEEE International Conference on Acoustics, Speech, and Signal Processing, Volume III, Digital Speech Processing, pp. 233-236.
Such transmission systems can be used for transmitting audio or video signals by means of a transmission medium like a radio channel, a coaxial cable or a glass fibre. It is also possible to use such transmission systems for recording of speech or video signals on a recording medium such as a magnetic tape or disc. Applications for such recording are automatic answering machined, digital video recorders or video servers for video on demand applications.
Digital transmission or recording of audio and video data involves huge amounts of bits to be transmitted or to be stored. To reduce these amounts of bits, numerous types of coders have been developed. Some of said coders use an analysis filter bank which derives at least two filtered signals from an input signal. Each of said filtered signals is converted into an encoded signal using a coding method suitable for it. In general, the sampling rate of the filtered signals will be reduced before encoding in order to keep the total number of samples representing the input signal(s) constant. The reduced bandwidth of each of the filtered signals allows such sampling rate reduction. The coding of the output signals of the filter bank can comprise a quantisation step followed by a lossless coding step.
The encoded signal is transmitted to a receiver which comprises decoding means which convert the encoded signals back into at least two signals. From these at least two signals a replica of the input signal of the transmitter is reconstructed by using a synthesis filter bank.
This approach allows the coding method used for each of the filtered signals to be adapted to the properties of said signals. It has turned out that the use of this approach results in an improved ratio between coding quality versus bitrate for certain types of signals.
In the system according to the above mentioned conference paper it is proposed to use time varying analysis and synthesis filter banks. The use of time varying filter banks has the advantage that the characteristics of the filter banks can be adapted to the input signal to be coded, resulting into further improved coding properties. By using time varying filter banks, transition phenomena can occur when the properties of the filter banks are changed in time. To reduce these transient phenomena, in the system according to the above mentioned conference paper so- called transition filters are used. These transition filters compensate for distortion of the signal to be encoded during the change from one bank of analysis filters to a different bank of filters.
Due to the presence of transition filters the number of changes in the filter banks per unit of time is limited. This means that for input signals having fast changing properties, such like video signals, the filter banks can not be adapted fast enough to follow the changing properties of the signal to be encoded.